Combustion at high temperature leads to the formation of NOx, or oxides of nitrogen, because of the combination of oxygen with nitrogen at high temperature. This is a notorious pollutant and much effort is being put forth to reduce the formation of NOx.
Current gas turbine engines used combustion systems in which the fuel is directly injected into the front end of the combustor. The result on the fuel-air mixture must assure stable efficient combustion. Where no attempt is made to premix these flows, wide variations in the mixture fuel-air ratio exists. Local regions having near stoichiometric fixtures create high temperature combustion products which generate the high NOx levels. In an effort to decrease the combustor emissions of NOx, advanced designs have focused on premixing the fuel and air prior to their introduction into the combustor. In this way both the occurrence of high temperature combustor regions, and the peak temperature within them are minimized. As a consequence, NOx formation is minimized.
Such a strategy is more easily executed for gas fuel devices because a change of phase of the fuel is not required, and the overall fuel-air mixing process can accelerated. With the use of liquid fuel, a high fuel-air ratio inherently exists at the liquid droplet an interface. The strategy must therefore achieve adequate levels of fuel atomization and vaporization simultaneous with fuel distribution and mixing processes. The strategy which relies on fuel-air premixing to suppress peak temperatures is a "dry" NOx control, which is contrast to "wet" NOx control which injects steam or water into the nozzle to suppress flange temperature.
It is desirable that combustion be maintained outside the fuel injector with no flashback or recirculation into the nozzle. The liquid fuel should be vaporized before discharging into the combustor at high power. Where the liquid fuel nozzle is combined with a gas nozzle, the good gas performance of the gas combustion should not be decreased. It is desirable that a uniform mix before ignition be achieved because too rich an area leads to High NOx generation.